Bleaching process with at least one extraction stage

ABSTRACT

An improved pulp bleaching process having at least one Do delignification stage followed by at least one extraction stage is selected from the group consisting of Eo, Eop, Ep, and E extraction stage. The process comprises treating the delignified pulp Do pulp from the Do stage in a M DE  mixing stage with a filtrate from at least one extraction stage after the Do stage prior to treatment of the delignified pulp in the least one extraction stage, preferably with interstage washing.

FIELD OF THE INVENTION

This invention relates to the bleaching process with at least one extraction stage. More particularly, the invention relates to decreasing bleaching chemical (ClO₂ and NaOH) charge and steam required in the bleaching extraction stage.

BACKGROUND OF THE INVENTION

The recycling of some of the extraction effluent during chlorination pulp bleaching is a common practice. See for example U.S. Pat. No. 5,352,332.

SUMMARY OF THE INVENTION

One aspect of this invention relates to an improved pulp bleaching process having at least one Do delignification stage followed by at least one extraction stage selected from the group consisting of Eo, Eop, Ep, and E extraction stage, the process comprises treating the delignified pulp Do pulp from the Do stage in a M_(DE) stage with a filtrate from the at least one extraction stage after the Do stage prior to treatment of the delignified pulp in the least one extraction stage, preferably with interstage washing, wherein:

Do is a delignification stage in which pulp is treated with an agent comprising chlorine dioxide preferably no or substantially no elemental chlorine dioxide (ECF);

E₀ is an extraction stage in which a pulp is extracted with base in the presence of oxygen;

E is an extraction stage in which a pulp is extracted with a base;

Eop is an extraction stage in which pulp is treated with a composition comprising oxygen, base, and peroxide;

Ep is an extraction stage in which pulp is treated with a composition comprising base and peroxide;

and

M_(DE) is a treatment stage in which delignified pulp is treated with a filtrate from the at least one extraction stage. The M_(DE) can be accomplished in the bottom dilution zone of the Do tower (for a down flow Do tower) or a pipe to provide contact and mixing of extraction filtrate and Do pulp slurry.

Another aspect of the invention relates to an improved pulp bleaching process having at least one D₀ delignification stage followed by at least one extraction stage selected from the group consisting of Eo, Eop, Ep, and E extraction stages which is followed by at least one D₁ bleaching stage, the process comprises treating the delignified pulp from the D₀ stage in a M_(DE) stage with a filtrate from the at least one extraction stage wherein D₀, Eo, Eop, Ep, and E are as defined above and D₁ is a bleaching stage in which pulp is bleached with a bleaching agent comprising chorine dioxide.

The process of the present invention provides one or more advantages over prior processes for brightening bleached pulps. For example, advantages of some of the embodiments of the process of this invention include 1) a reduction in the amount of base used in the at least one bleaching stage, 2) reducing the steam/energy consumption, 3) ClO₂ reduction in the D₁ stage, 4) lower at least one extraction stage pulp kappa number and less carryover in the at least one extraction stage, 5) reduce Eop filtrate discharge at least one extraction stage, 6) reduction in filtrate AOX and color reduction or 7) a combination of two or more of the aforementioned advantages. Some embodiments of this invention may exhibit one of the aforementioned advantages while other preferred embodiments may exhibit two or more of the foregoing advantages in any combination.

BRIEF DESCRIPTION OF THE DRAWING

A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawing in which:

FIG. 1 is a schematic illustration of the overall pulp making in accordance to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

While this invention is susceptible of embodiment in many different forms, there is shown and described in drawing, figures, and examples and will herein be described in detail preferred embodiments of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspect of the invention to the embodiments illustrated.

One aspect of this invention relates to an improved pulp bleaching process having at least one D₀ delignification stage followed by at least one extraction stage selected from the group consisting of Eo, Eop, Ep, and E extraction stage. The process comprises treating the delignified pulp D₀ pulp from the D₀ in a M_(DE) stage with a filtrate from the at least one extraction stage after the D₀ stage prior to treatment of the delignified pulp in the least one extraction stage, preferably with an interstage washing, wherein D₀, Eo, E, Eop, Ep, and M_(DE) are as defined above.

M_(DE) is a mixing stage in which the pulp is mixed with a filtrate. The method and apparatus of the present invention can be varied widely, for example, the mixing stage may be conducted in a separate tower at a point between the chlorination washing step and the step of mixing the pulp with an alkaline extracting solution. Alternatively, the M_(DE) can also be accomplished in the dilution zone of the bottom of the D₀ tower (for a down flow D₀ tower) or a pipe to provide contact and mixing of extraction filtrate and D₀ pulp slurry.

The pH of the M_(DE) stage can be varied widely provided the pH range is greater than 2.5. Any pH within this range can be used. For example, the pH can be as high as about 4.5 and as low as about 2.5 to about 3.5. In the preferred embodiments of the invention, the pH is greater than or equal to 3. In the more preferred embodiments of the invention, the pH is from about 3.5 to about 6.0 and in the most preferred embodiments of the invention, the pH is from about 3.5 to about 4.5.

The consistency (CSC) of the pulp in the M_(DE) stage may vary widely and any consistency that provides the desired increase in pulp brightness may be used. The pulp may be bleached under low consistency conditions (i.e. from about 3 to about 5 based on the total weight of the mixture of pulp and bleaching chemicals), medium consistency conditions (i.e. from about 8 to about 15 based on the total weight of the mixture of pulp and bleaching chemicals) or high consistency conditions (i.e. from about 25 to about 30 based on the total weight of the mixture of pulp and bleaching chemicals). The consistency is preferably from about 5 to 30, more preferably from about 10 to 20, and most preferably from about 10 to about 15.

The M_(DE) stage retention times will vary widely and retention times used in the conventional bleaching stages may be used. Usually, retention times will be at least about 2 minutes. Retention times are preferably from about 2 min. to about 120 min., and are more preferably from about 5 minutes to about 60 min. and most preferably from about 5 min. to about 15 min.

Similarly, The M_(DE) stage treatment temperatures employed in the critical treatment stage may vary widely and temperatures employed in the conventional bleaching stages may be used. For example, useful temperatures can be as low as about 50° C. or lower and as high as about 80° C. or higher. In the process of this invention, the treatment temperature is usually from about 50° C. to about 85° C., preferably from about 60° C. to about 80° C., more preferably from about 65° C. to about 75° C. and most preferably from about 65° C. to about 70° C.

After M_(DE) stage, the pulp is preferably washed or is washed at some subsequent point in the process prior to the Ep or Eop stage. Pulp washing after the M_(DE) stage removes the acidic species (NaHCO₃ and dissolved CO₂) to minimize its caustic demand in the Eop stage.

After the washing stage, the pulp is subjected to extraction in an E, E₀, Eop or Ep stage. The Eop stage refers to alkaline extraction of pulp with oxygen (O) and hydrogen peroxide (P), and Ep stage refers to alkaline extraction of pulp with hydrogen peroxide (P). Conventional processes and apparatus can be used in the Ep or Eop stage. See for example “Pulp Bleaching Principles and Practice of Pulp Bleaching” Carlton W. Dence and Douglas W. Reeve, TAPPI Press, 1996 and references cited therein.

An advantage of a preferred embodiment of this invention is the reduction of extraction chemicals such as NaOH in the Eop and/or Ep stages as compared to the same or substantially the same bleaching processes which do not include the M_(DE) stage. For example, the reduction in the amount of NaOH is typically at least about 10%, preferably at least about 20%, more preferably from about 15% to about 30% and most preferably from about 20% to about 25% less than the amount of NaOH used in the same or substantially the same bleaching processes which do not include the M_(DE) stage.

The amount of extraction agent used (e.g. sodium hydroxide, magnesium hydroxide, potassium hydroxide, etc.) used in the practice of the process of this invention can vary widely and any amount sufficient to provide the desired lignin extraction efficiency and the desired degree of brightness can be used. The amount of caustic used is usually at least about 0.5% based on the dry weight of the pulp. Preferably the amount of bleaching agent is from about 1% to about 8%, more preferably from about 1.5% to about 3% and most preferably about 1-2% on the aforementioned basis.

Another advantage of a preferred embodiment of this invention is the reduction of AOX resulting from the Eop and/or Ep stages as compared to the same or substantially the same bleaching processes which do not include the M_(DE) stage. For example, the reduction in the amount of AOX is typically at least about 20%, preferably at least about 30%, more preferably from about 10% to about 50% and most preferably from about 20% to about 30% less than the amount of AOX produced in the same or substantially the same bleaching processes which do not include the M_(DE) stage to obtain the same or substantially level of pulp brightness in the Eop and/or Ep stages.

The bleaching process of this invention may include other bleaching stages as for example bleaching with Cl₂, peroxy acids, chlorine dioxide, ozone and the like, and extraction stages such as extraction with oxygen, ozone, borohydride, chlorine dioxide and the like in the presence other bases such as Mg(OH)₂. Illustrative of such bleaching processes are OD₀/C M_(DE)Eop, D₀ M_(DE)EopD, D₀ M_(DE) EOPDD, D₀ M_(DE) EopED, D₀ M_(DE) EDEpEopD, ZE M_(DE) (Eop)D, Z M_(DE) (Eop)D, D₀ M_(DE) EpZ(Eop), D₀ M_(DE) EpZD(ZD), D₀ M_(DE) (Eop)D(ZD), D₀ M_(DE) (Eop)PP, D₀ M_(DE) (Eop)DZ, D₀ M_(DE) EopD₁, ODo M_(DE) EopD₁, Do M_(DE) EopD₁D₂, ODo M_(DE) EopD₁D₂, Do M_(DE) EopD₁EpD₂, ODo M_(DE) EopD₁EpD₂, D M_(DE) EopD₁P and the like in which D is as described above and Z is ozone, E is extraction in the presence of base, O is oxygen, P is peroxide, D/C is a mixture of chlorine dioxide and elemental chlorine and two or more symbols in parenthesis indicate an absence of an intermediate washing stage. The Eop, Ep, and Do are defined above herein. The processes and apparatus used in the D, Z, E, 0, P, D/C are conventional and there are well known in art. See for example, “Pulp Bleaching Principles and Practice of Pulp Bleaching” Carlton W. Dence and Douglas W. Reeve, TAPPI Press, 1996 and references cited therein.

In the preferred embodiment of the invention, the process comprises at least one D₁ bleaching stage following the extraction stage. Another advantage of a preferred embodiment of this invention is the reduction of bleaching chemicals such as ClO₂, H₂SO₄, or combination thereof in the D₁ stage as compared to the same or substantially the same bleaching processes which do not include the M_(DE) stage. For example, the reduction in the amount of ClO₂ is typically at least about 5%, preferably at least about 10%, more preferably from about 5% to about 20% and most preferably from about 10% to about 20% less than the amount of ClO₂ used in the same or substantially the same bleaching processes which do not include the M_(DE) stage to obtain the same or substantially level of pulp brightness in the Eop and or Ep stages.

The plant source of pulp for use in this invention is not critical and may be any fibrous plant which can be subjected to chemical pulp bleaching. Examples of such fibrous plants are trees, including hardwood fibrous trees such as aspen, eucalyptus, maple, birch, walnut, acacia and softwood fibrous trees such as spruce, pine, cedar, including mixtures thereof. In certain embodiments, at least a portion of the pulp fibers may be provided from non-woody herbaceous plants including, but not limited to, kenaf, hemp, jute, flax, sisal, or abaca although legal restrictions and other considerations may make the utilization of hemp and other fiber sources impractical or impossible. The source of pulp for use in the practice of this invention is preferably hardwood and softwood fibrous trees, more preferably Eucalyptus, Spruce and Aspen and is most preferably Aspen and Spruce.

The pulp used in the process of this invention can be obtained by subjecting the fibrous plant to any chemical pulping process. Following the wood digestion process, pulp is separated from the spent pulping liquor. The spent pulping liquor is then recovered and regenerated for recycling. The pulp is then bleached and purified in a bleach plant operation.

The pulp of this invention can also be used in the manufacture of paper and packaging products such as printing, writing, publication and cover papers and paperboard products. Illustrative of these products and processes for their manufacture are those described in U.S. Pat. Nos. 5,902,454 and 6,464,832.

For example, in the paper or paperboard making process, the bleached pulp of this invention or pulp mixtures comprising the bleached pulp of this invention is formulated into an aqueous paper making stock furnish which also comprises one of more additives which impart or enhance specific sheet properties or which control other process parameters. Illustrative of such additives is alum which is used to control pH, fix additives onto pulp fibers and improve retention of the pulp fibers on the paper making machine. Other aluminum based chemicals which may be added to furnish are sodium aluminate, poly aluminum silicate sulfate and poly aluminum chloride. Other wet end chemicals which may be included in the paper making stock furnish for conventional purposes are acid and bases, sizing agents, dry-strength resins, wet strength resins, fillers, coloring materials, retention aids, fiber flocculants, defoamers, drainage aids, optical brighteners, pitch control chemicals, slimicides, biocides, specialty chemicals such as corrosion inhibitors, flame proofing and anti-tarnish chemicals, and the like.

The aqueous paper making stock furnish comprising the bleached pulp and the aluminum based compounds is deposited onto the forming wire of a conventional paper making machine to form a wet deposited web of paper or paperboard and the wet deposited web of paper or paperboard is dried to form a dried web of paper or paperboard. Paper making machines and the use of same to make paper are well known in the art and will not be described in any great detail. See for example, Pulp and Paper Chemistry and Handbook for Pulp & Paper Technologies, supra. By way of example, the aqueous paper making stock furnish containing pulp, aluminum based and other optional additives and usually having a consistency of from about 0.3% to about 1% is deposited from the head box of a suitable paper making machine as for example a twin or single wire Fourdrinier machine. The deposited paper making stock furnish is dewatered by vacuum in the forming section. The dewatered furnish is conveyed from the forming section to the press section on specially-constructed felts through a series of roll press nips which removes water and consolidates the wet web of paper and thereafter to the dryer section where the wet web of paper is dried to form the dried web of paper of this invention. After drying, the dried web of paper may be optionally subjected to several dry end operations such as and various surface treatments such as coating, and sizing and calendering.

The paper manufactured in accordance with this invention can be used for conventional purposes. For example, the paper is useful as printing paper, publication paper, newsprint and the like.

The present invention is described in more detail by referring to the following examples and comparative examples which are intended to more practically illustrate the invention and not to be a limitation thereon.

EXAMPLE 1

FIG. 1 illustrates a portion of a bleach plant 10 that is used to produce bleached pulp in accordance with the preferred embodiment of the invention. The unbleached pulp 12 is conveyed to a low density chest 14 via line 16. In the low density chest 14, the unbleached pulp 12 is further diluted with water and then the pulp is mixed with ClO₂ in the mixer 18 before the pulp 12 is transferred to down flow Do delignification 22 tower via line 20. In the Do delignification 22 tower, lignin is oxidized and then the pulp 12 is transferred to washer 24 via lines 26 to remove oxidized lignin and inorganic materials. After the last Do washing stage 24, the pulp preferably has a consistency of from about 8% to about 15%. The pulp 12 is then transferred through a heat exchanger 36 via line 34 and then to the extraction tower 30 with peroxide (Eop). After, the Eop stage, the pulp 12 can be stored in a storage tank (not depicted) until required for the first acidic bleaching stage D₁ 50. In the preferred embodiment of the invention, the pulp 12 is transferred from the extraction stage 40 to a second washer 30 via line 40. All or a portion of the filtrate from the washer 38 is transferred to filtrate tank 42, via line 44. The filtrate tank 42 can be stored for future use or can be conveyed via line 46 to the dilution zone 48 at the bottom of delignification tower 22 wherein the filtrate is mixed with pulp 12 after the pulp has been delignified in the delignification tower 22. Although not depicted, delignified pulp 12 can also be mixed with the extraction stage filtrate outside of the delignification tower 22, as for example, in a pipe connecting delignification tower 22 to washer 24, or at any other point in the process between delignification tower 22 and extraction tower 30.

The extraction stage filtrate and the D stage delignified pulp 12 are preferably mixed at a pH greater 2.5 and more preferably at pH 3.5 to 6.0. The consistency of the pulp and extraction filtrate mixture can be varied widely, but the consistency of the pulp is preferably from about 10 to 20.

The retention time (t) can be varied widely, but the retention time can be from about 5 to 60 minutes and most preferably the retention time (t) is from about 5 to 15 min. The treatment temperature (T) can be also varied widely, but preferably the treatment temperature is from 60° C. to 80° C. and most preferably is from 65° C. to 70° C.

After the extraction filtrate and the D0 pulp are mixed for a desired period of time, the pulp is preferably washed prior to transfer to the extraction stage to remove acidic species such as NaHCO3, and dissolved CO2) to minimize caustic demand in the extraction stage. For example, as depicted in FIG. 1, the pulp is washed in washer 24 before transferred to the extraction tower 30. After washing the pulp in the 2^(nd) washer 38, the washed pulp is conveyed via 52 to mixer 54 wherein the pulp is mixed with the ClO2 before the pulp is transferred to the 1^(st) acidic bleaching stage tower 50. In first acidic bleaching stage 50, the pulp 12 is bleached under acidic conditions with a bleaching agent comprising chlorine dioxide. In the preferred embodiments of the invention as depicted in the FIG. 1, the bleaching agent is chlorine dioxide comprising less than about 1.5%, preferably less than about 1%, more preferably less than about 0.5% and most preferably less than about 0.3% of the active bleaching agent is elemental chlorine. In the embodiments of the invention of choice, the active bleaching agent is chlorine dioxide which contains no or substantially no elemental chlorine (i.e. less than about 1% to about 5%). The application rates, pHs, times and temperatures used in the acidic bleaching stage may vary widely and any known to the art can be used. The bleached pulp 12 is conveyed via line 56 to at least one post first acidic bleaching stage washer or decker 58.

The pulp can be processed from system and used for conventional purposes or the pulp can be subjected to one or more additional acidic and/or alkaline bleaching stages either before or after the first acidic bleaching, alkaline bleaching stage and/or second acidic bleaching stage. As for example, further pulp bleaching with one or more bleaching agents selected from the group consisting of peroxide, chlorine dioxide and ozone. Such additional bleaching stages may be without subsequent washing or may be followed by subsequent wash stage or stage(s). The washed pulp exits the bleaching sequence via line 46 for conventional use as for example in a paper making process.

EXAMPLE 2 Hardwood Pulp Study

Unbleached pulps and Eop filtrates were supplied by Mill A. The unbleached hardwood pulp had 24.6% brightness and 11.7 P#. The unbleached softwood pulp had 24.6% brightness and 13.4 P#. The pHs of the Eop filtrates were unusually higher at 10.4 and 11 for hardwood and softwood bleach lines, respectively. The 200 OD gram of the Do pulp along with Do filtrate at 3.5% consistency was treated with 2 liter of Eop filtrate, assuming the availability of 2640 gallon Eop filtrate per ton of pulp for recycling.

All bleaching except for the Eop stage was conducted in sealed plastic bags. All pulp samples were preheated to the bleaching temperature, and all the chemicals were added sequentially and mixed thoroughly with the pulp before addition of another chemical. The chemical addition sequence in the D stages are deionized water, caustic (for pH control), and ClO₂.

After completing the D₁ bleaching stage, the pulp was squeezed to collect filtrate for pH, residual, and COD measurement. The pulp was repulped at 1% consistency with deionized water and dewatered on a Buchner funnel and repeat a couple of time to simulate a pulp washing stage in mills. The washed pulp was analyzed for brightness, viscosity, and pulp dirt. All the filtrate and pulp analysis was done with the standard published procedures understood by all the people working in the field.

The Eop stage was simulated in a pressurized reactor where O₂ pressure can be applied to simulate the mill practice. The Quantum reactor widely known to all in the field was used for lab simulation of the Eop stage.

Initial experiments were conducted at three different temperatures for 30 min of treatment time. The effect of Eop filtrate preextraction of Do pulp on Do brightness is shown in Table 1. The 3-4% brightness increase was observed, depending upon the treatment temperature. The pretreated Do pulp pHs were at 3.9 compared with the control Do pulp pH of 2.82.

TABLE 1 Preextraction of HW Do pulp with Eop Filtrate - Effect of Treatment Temperature Mill A Hardwood Pulp: 24.6% brightness and 11.7 P# Extraction temperature, ° C. No treatment 55 65 75 Do pH 2.82 3.96 3.91 3.9 Do Brightness, % 37.7 39.2 40.4 41.0

The Do pulps were then subject to normal Eop treatment at the conditions simulating those in Mill A operation. The results are shown in Table 2. All the Eop pHs were lower than normal mill Eop pH target because of insufficient caustic charge and more H₂O₂ usage in the lab than in mill practice. The caustic charges for preextracted Do pulps were 4 lb/t less than that of the control by design. The Eop end pHs for extracted Do pulps were slightly higher than that of the control Eop because of the removal of the neutralized acidic chloride and organic acids in the Eop stage. The Eop treatment efficiency is higher for the preextracted Do pulp than the control Do pulp, manifested by a 10% lower Eop P#. For the unknown reasons, the Eop brightness of preextracted pulps was lower than that of the control.

TABLE 2 Prextraction of HW Do pulp with Eop Filtrate - Effect of Treatment Temperature Mill A Hardwood Pulp: 24.6% brightness and 11.7 P# Extraction temperature, ° C. No treatment 55 65 75 Do Brightness, % 37.7 39.2 40.4 41.0 Eop NaOH, % 1.2 1.0 1.0 1.0 Eop End pH 9.9 10 9.91 9.93 Eop Brightness, % 63.4 60.4 61.6 61.9 Eop P# 3.8 3.4 3.3 3.3 D1 pH 3.01 3.09 2.96 2.94 D1 Brightness, % 80.5 80.9 80.9 80.8 Eop conditions: 10% CSC, 85° C., 45 min, 0.5% H₂O₂ D₁ conditions: 10% CSC, 70° C., 120 min, 0.6% ClO₂

Lower Eop pulp P# should lead to some ClO₂ reduction in the D₁ stage as the ClO₂ bleachability in the D₁ stage is dependent upon the Eop pulp P#. Lab bleaching at the same 0.6% ClO₂ charge shows 0.4% higher D₁ brightness for Eop filtrate preextracted pulps than the control at the unoptimized D₁ pHs (Table 2). Based on the 10% lower Eop pulp P#, greater benefits in terms of D₁ stage ClO₂ savings or D₁ brightness increase are expected for Eop filtrate preextracted pulps than the control. Perhaps, there is some lignin reprecipitation occurring when bringing the Eop filtrate down to the acidic pH.

EXAMPLE 2 Effect of Treatment Time

The effect of treatment time was subsequently investigated to determine if the treatment can be accomplished in the existing pipe from the top or bottom of the Do tower to Do washer vat without additional retention tower. The results are shown in Table 3. Treatment time at 30 and 2 minutes produced similar results except for a slightly higher Eop P# at 2 minute treatment time than at 30 minutes, consistent to the expected less efficient lignin preextraction by Eop filtrate at shorter time. Among the three-tier caustic demands, neutralization of chloride and organic acids in the Do pulp and filtrate are inorganic reactions and should be largely completed within a minute or so available in the pipe. Therefore, the premise of using Eop filtrate for neutralizing the Do pulp and filtrate for Eop caustic reduction and delignification efficiency enhancement for D₁ ClO₂ reduction is achievable at the short time available with the existing equipment.

TABLE 3 Preextraction of HW Do pulp with Eop Filtrate - Effect of Treatment Time Mill A Hardwood Pulp: 24.6% brightness and 11.7 P# Treatment time, min No Treatment 30 2 Do Brightness, % 37.7 40.4 39.7 Eop NaOH, % 1.2 1.0 1.0 Eop End pH 9.9 9.91 9.66 Eop Brightness, % 63.4 61.6 62.2 Eop P# 3.8 3.3 3.5 Eop filtrate treatment conditions: 65° C. and 2 liters on 200 OD gram pulp

EXAMPLE 3 Softwood Pulp Study

Similar results are achieved in preextraction of softwood Do pulp and filtrate with softwood Eop filtrate (Table 4). The results for softwood pulp in Table 4 validate those achieved for HW. Therefore, the strategy is potentially feasible for both species to achieve 4 lb/t caustic savings in the Eop stage and some ClO₂ reduction in the D₁ stage due to lower Eop pulp P#.

TABLE 4 Preextraction of SW Do pulp with Eop Filtrate - Effect of Treatment Temperature Mill A Softwood Pulp: 24.6% brightness and 13.4 P# Extraction temperature, ° C. No treatment 55 65 75 pH 2.68 4.21 4.28 4.09 Do Brightness, % 34.4 32.1 33.8 33.1 Eop NaOH, % 1.5 1.3 1.3 1.3 Eop End pH 10.17 10.23 10.1 10.1 Eop Brightness, % 58.5 56.5 57.7 57.8 Eop P# 2.5 2.3 2.1 2.1 Eop conditions: 10% CSC, 85° C., 45 min, 0.5% H₂O₂

EXAMPLE 4 Effect of Treatment Time and the Amount of Eop Filtrate

The results in Table 5 show that reduced treatment time (from 30 mins versus 2 mins) and the amount of Eop filtrate (from 2 liter versus 1 liter) in the Eop filtrate preextraction of Do pulp and filtrate still allow about the same (4 lb/t) caustic usage savings in the Eop stage with enhanced Eop extraction efficiency. The Eop filtrate preextracted Do pulp had higher Eop brightness and lower P# than the control. The lower Eop P# of the Eop filtrate preextracted pulp is translated to better D1 bleachability measured by higher D1 brightness. The Eop filtrate preextraction of Do pulp and filtrate had very limited effect on pulp viscosity and dirt removal (Table 5).

TABLE 5 Effect of Treatment Time and The Amount of Eop Filtrate Mill A Hardwood Pulp: 23.7% brightness and 15.3 P# Treatment time, min No Treatment 30 2 2 Eop Filtrate, 0 2 2 1 liter Do pH 2.39 3.33 3.25 2.82 Do Brightness, 30.1 30.1 30.9 31 % Eop NaOH, % 1.5 1.3 1.3 1.3 Eop End pH 10.36 10.1 9.9 9.29 Eop 47.7 50.5 49.8 46.7 Brightness, % Eop P# 4.2 3.6 3.9 3.9 Eop Viscosity, 23.5 22.8 23.1 23.1 cPs D1 ClO₂, % 0.4 0.8 0.4 0.8 0.4 0.8 0.4 0.8 D1 pH 3.24 2.71 3.34 2.75 3.43 2.71 3.26 2.7 D1 Brightness, 67.2 72.7 68.6 76.1 66.8 74.7 69.9 75.0 % D1 Viscosity, 23.3 22.9 22.7 22.0 22.7 22.1 22.1 22.1 cPs D1 Dirt, ppm 1.65 0 2.25 0.86 1.14 0.21 0.96 0.2 Eop filtrate treatment conditions: 65° C. and 200 OD gram pulp

Brightness

Approximately 5 grams of pulp is rolled or pressed on a disc and is permitted to completely dry. The brightness is measured on both sides of the brightness pad, at least four readings per side and then the average is calculated. These readings are performed on a GE brightness meter which reads a directional brightness or on an ISO brightness meter which reads a diffused brightness. Both instruments are made by Technidyne Corp.

Reverted Brightness

Reverted brightness, a standard lab test for pulp brightness stability, was conducted by placing the pulp brightness pad (after brightness reading) in an oven at 105° C. for 60 min. After that, the brightness pad is read for brightness as reverted brightness.

Viscosity

The viscosity is a measurement used to compare a relative strength property of the pulp.

This property is used to determine the percentage of hardwood/softwood for making different grades of paper. A Cannon-Fenske (200) viscometer tube, calibrated for 25° C., is used for testing bleached pulps. The sample size is 0.2000 grams, using 20 ml, 1.0 molar CED and 20 ml DI water mixed thoroughly to break down the pulp fiber.

Permanganate Number

The Permanganate Number indicates the amount of lignin that is in the pulp. (The Kappa number is generally used only on the brownstock, while the value for the Permanganate Number is comparative to the bleached pulp.) The procedure for determining the Permanganate Number is:

-   -   1. Weigh exactly 1.00 gram sample.     -   2. Put the sample in a blender with 700 ml DI water and blend         about 45 seconds, pour the sample into a battery jar on a stir         plate.     -   3. Add exactly 25 ml of 0.1 N Potassium Permanganate and 25 ml         4N H₂SO₄, starting a timer set for 5 min.     -   4. When the timer stops, add 6 ml 1 Molar KI and allow it to mix         thoroughly to kill the reaction.     -   5. Titrate to a starch end point with 0.1N Sodium Thiosulfate.         Record mls titrated.     -   6. In 700 ml DI water without the pulp sample, use the same         reagents and titrate to use as a blank. Using an accurately         prepared Potassium Permanganate, the blank should be 25.0     -   7. Subtract the mls titrated with the sample from the mls         titrated for the blank and the result will be the P Number.

Dirt

Pulp dirt count is done by a visual count of all the dirt spots on the brightness pad and is the size weighted sum of the total dirt spots according to a Tappi temperature rate.

Various modifications and variations may be devised given the above-described embodiments of the invention. It is intended that all embodiments and modifications and variations thereof be included within the scope of the invention as it is defined in the following claims. 

1. A pulp bleaching process comprising at least one D₀ stage and at least one extraction stage selected from a group consisting of Eo, Eop, Ep, and E extraction stage, the process comprising: treating a D₀ pulp from the D₀ in a M_(DE) mixing stage with a extraction filtrate from the at least one extraction stage prior to treatment of the D₀ pulp in the least one extraction stage wherein: Do is a delignification stage in which pulp is treated with an agent comprising chlorine dioxide preferably no or substantially no elemental chlorine dioxide (ECF); E₀ is an extraction stage in which a pulp is extracted with base in the presence of oxygen; E is an extraction stage in which a pulp is extracted with a base; Eop is an extraction stage in which pulp is treated with a composition comprising oxygen, base, and peroxide; Ep is an extraction stage in which pulp is treated with a composition comprising base and peroxide; and M_(DE) is a mixing stage in which delignified pulp is treated with a filtrate from the at least one extraction stage. The M_(DE) can be accomplished in the bottom dilution zone of the D₀ tower (for a down flow D₀ tower) or a pipe to provide contact and mixing of extraction filtrate and D₀ pulp slurry.
 2. The pulp bleaching process of claim 1 wherein the M_(DE) mixing stage is conducted in a separate tower at a point between a chlorination washing step and a step of mixing the pulp with an alkaline extracting solution.
 3. The pulp bleaching process of claim 1 wherein the M_(DE) mixing stage is conducted in the dilution zone at bottom of a down flow D₀ tower or a pipe to provide contact and mixing of the extraction filtrate and D₀ pulp.
 4. The pulp bleaching process of claim 1 wherein the pulp has a pH of greater than 2.5.
 5. The pulp bleaching process of claim 1 wherein the pulp in the M_(DE) mixing stage has a pH of from about 4.5 and to about 2.5.
 6. The pulp bleaching process of claim 1 wherein the pulp in the M_(DE) mixing stage has a pH from about 3.5 to about 6.0.
 7. The pulp bleaching process of claim 1 wherein the pulp in the M_(DE) mixing stage has a pH from about 3.5 to about 4.5.
 8. The pulp bleaching process of claim 1 wherein the pulp in the M_(DE) mixing stage has a consistency from about 5 to about
 30. 9. The pulp bleaching process of claim 1 wherein the pulp in the M_(DE) mixing stage has a consistency from about 10 to about
 20. 10. The pulp bleaching process of claim 1 wherein the pulp in the M_(DE) mixing stage has a consistency from about 10 to about
 15. 11. The pulp bleaching process of claim 1 wherein the pulp in the M_(DE) mixing stage has a retention time of at least about 2 minutes.
 12. The pulp bleaching process of claim 1 wherein the pulp in the M_(DE) mixing stage has a retention time from about 2 minutes to about 120 minutes.
 13. The pulp bleaching process of claim 1 wherein the pulp in the M_(DE) mixing stage has a retention time from about 5 minutes to about 60 minutes.
 14. The pulp bleaching process of claim 1 wherein the pulp in the M_(DE) mixing stage has a retention time from about 5 minutes to about 15 minutes.
 15. The pulp bleaching process of claim 1 wherein the pulp in the M_(DE) mixing stage has a treatment temperature from about 50° C. to about 85° C.
 16. The pulp bleaching process of claim 1 wherein the pulp in the M_(DE) mixing stage has a treatment temperature from about 60° C. to about 80° C.
 17. The pulp bleaching process of claim 1 wherein the pulp in the M_(DE) mixing stage has a treatment temperature from about 65° C. to about 70° C.
 18. The pulp bleaching process of claim 1 further comprising bleaching the pulp from the extraction stage in at least one D₁ bleaching stage, the process comprising chorine dioxide.
 19. The pulp bleaching process of claim 1 wherein an amount of NaOH used in the extraction stage is reduced as compared to an amount of NaOH used in the same or substantially the same bleaching processes which do not include the M_(DE) mixing stage.
 20. The pulp bleaching process of claim 19 wherein the reduction in the amount of NaOH is from about 15% to about 30% less than the amount of NaOH used in the same or substantially the same bleaching processes which do not include the M_(DE) mixing stage.
 21. The pulp bleaching process of claim 18 wherein an amount of ClO₂ used in the D₁ bleaching stage is reduced as compared to the amount of ClO₂ used in the same or substantially the same bleaching processes which do not include the M_(DE) mixing stage.
 22. The pulp bleaching process of claim 21 wherein a reduction in the amount of ClO₂ is from about 5% to about 20% less than the amount of ClO₂ used in the same or substantially the same bleaching processes which do not include the mixing stage (M_(DE)) to obtain the same or substantially level of pulp brightness.
 23. The improved pulp bleaching process of claim 1 wherein an amount of AOX resulting from the extraction stage is reduced as compared to the amount of AOX used in the same or substantially the same bleaching processes which do not include the M_(DE) mixing stage.
 24. The improved pulp bleaching process of claim 23 wherein the reduction in the amount of AOX is from about 10% to about 50%. 